Exhaust oxygen sensors have been used for many years in vehicles to sense the presence of oxygen in the exhaust gas from the vehicle engine. The signal from the exhaust oxygen sensor may be used to provide a feedback signal to control the amount of fuel injected into the engine to minimize the production of undesirable exhaust emissions. The exhaust sensor must reach a certain minimum temperature in order to produce a usable signal. In order to minimize the time required for the sensor to reach its operating temperature, the industry trend has been to move toward planar sensing elements with co-fired electrodes and integral heating circuits. Due to their reduced size, planar sensors are capable of shorter time to activity, i.e. time required to reach a temperature at which a usable signal is available, compared to earlier conical sensing elements. It has been found that planar sensing elements are more susceptible to thermal shock cracking due to contact with water droplets that may be present in the vehicle exhaust system.
Accordingly, a need exists in the sensor art for sensors that are less susceptible to thermal shock cracking due to contact with water droplets.